1. Field of the Invention
The present invention relates to ink jet printing apparatuses and image processing methods, and specifically, to a technique for reducing, in an image containing a character and its background, the blurredness and bleeding of characters.
2. Description of the Related Art
Ink jet printing apparatuses, which can perform printing to various print media, have the advantage of being able to perform dense and fast printing and have thus been used as information output devices for various apparatuses. These devices have been commercialized as printers, portable printers, and copiers.
What is called a serial type of an ink jet printing apparatus comprises a carriage mounting a print head having a plurality of ink ejection openings and an ink tank, a conveying mechanism that conveys a print medium, and control means for controlling the operation of the carriage and the conveying mechanism. This ink jet printing apparatus performs scans of the print head in a direction (main scanning direction) orthogonal to the direction of conveyance of a print medium (sub-scanning direction). Between these scans, the print medium is conveyed by an amount equal to a print width of printing. This ink jet printing system performs printing by ejecting ink to the print medium in accordance with print signals. The ink jet printing system is widely used, owing to its low running costs and quiet operations. In recent years, many products use a plurality of colors of ink so as to be adaptable to color printing.
On these color ink jet apparatuses, it is known that in an image composed of a character and its background, the character may become blurred or have bleeding. More specifically, first, in an image in which the character and its background are composed of different colors from each other, the inks of the different colors may bleed and mix with each other at the boundary between the character and background. This may degrade the visibility of the character. Second, in an image composed of an outline character against a colored background, the background color ink may bleed into the outline character area to blur the outline character, degrading their visibility.
Measures described below have been proposed to deal with the above problems.
A first measure is to provide a fixing mechanism such as a heating fixing device. This allows ink to be quickly fixed to a print medium to prevent bleeding, improving the visibility of characters. As a second measure, a proposal has been made of a method of detecting the boundary between a black image and a color image and applying a color ink on the black ink adjacent to the boundary so that the high permeability of the color ink serves to reduce possible boundary bleeding (see Japanese Patent Laid-Open No. 2005-169753). As a third measure, a technique is known which increases the permeability of inks through the print medium to prevent different inks from mixing together on the surface of the print medium (see Japanese Patent Laid-Open No. 55-65269 (1980)).
However, these methods have problems as described below.
On the first measure, the provision of the fixing mechanism may draw an increase of size and cost of the apparatus. Further, the serial type printer conveys the print medium for each scan, resulting in intermittent sheet conveying. Thus, uneven fixation may occur following the intermittent sheet conveying. Further, on the second measure in which the color ink is applied on the black ink adjacent to the boundary to reduce bleeding, in many cases, sufficient effects are not expected unless the black ink and the color ink have the properties of reacting with each other.
The third measure can prevent the different inks from mixing together on the print medium by increasing their permeability. However, increasing permeability may cause the inks to permeate a shallow layer in the print medium along its surface and move across the boundary. In particular, if an outline character is printed, such bleeding may degrade the visibility of the character.